A variation of sorcery
The nucleus of this system is similar to the sorcerer rules of the d20 OSR or the magic system of Adventurer Conqueror King System and D&D 5th edition, in brief each spell-caster has a reserve of stored Magical Energy Slots that can be used to cast Ready Spells. The number of Magical Energy Slots and of Ready Spell is dependent by the Spell-Caster level, for number of spells consult "Energy Slots Table", or your normal memorization table (ready spells are 2 more of the slots usable).
|Energy Slots Table|
Differently from other system Ready Spells can be changed with time, money and effort (1 day per spell level + 100 gp per spell level to be consumed in spell preparation if the spell book is at hand, otherwise the time is 1 week * spell level and 1000 gp per spell level, somehow similar to the egregious Adventurer Conqueror King System), once readied spells stay readied unless something bad happen to the spell-caster (death and resurrection, head trauma, brought to 0hp with head injuries and the like), in which case for each spell the spell caster must make a saving throw or lose the spell that will have to be readied again.
The biggest difference with other systems is the way Magical Energy Slots are recovered, the idea is that like in spell point system as time passes spell slots slowly recharge, the time can be counted only from the last time a spell was used (even a lowly cantrip), so spell-caster should try to not use magic when is not necessary (unless they don't need to recover higher level spells); this method will give spell-caster almost always a little number of cantrips and low level spells energy slots during the day but they will need to rest and don't use magic to recover high level spell slots, this is a mix of OSRIC/AD&D 1st time to memorize spells and more recent games that with 1 night of sleep give back all energy slots to a spell-caster, for times consult "Energy Slots Recovery Table".
|Energy Slots Recovery Table|
Another difference is the presence of 0th level spells that works like other spells (they have not infinite uses and which cantrip are Ready can be changed as with spells), this is more similar to the original idea of cantrips, you can ready a cantrip in 1h and 10 gp if you have a spell book at hand, double that w/out a spell book.
This variations can be freely adopted in any old school game.
So for example a 1st level Mage will be able to ready 5 cantrips and 3 1st level spells and he will have 3 0th and 1 1st level energy slots. If he don't cast any spell for 15' minutes he will recover 2 cantrips energy slots, after 30' 3 cantrips and 1 1st level energy slots, after 1h 3 cantrip and 2 1st but since he only have 1 1st level it can use the 2nd 1st level to recover another cantrip.
It's a kind of magic
Another particularity of the Castles & Chimeras magic system is that there is only one kind of full spell-caster, but he must chose which kind of magic he will use:
White magic: good at protection, banishment, destroyng undead, healing, recovery, strenghtening, divination, resurrection (cleric spell list revised with additions)
Elemental/Gray magic: good at elemental manipulation, transformation (mage spell list revised with additions)
Black magic: summoning, death and decay, poisoning, cursing (must be selected but the idea is clear)
Nature magic: druid spell list revised with additions (weather control, earthquakes, animal friend/speak, tree control and so on; some polymorphing - mostly animals, some regeneration, reincarnation)
Body Magic: polymorphing, regeneration (different kind of healing, see the Occult Physician on this blog)
Prismatic magic: illusions, shadows (illusionist spell list revised with additions)
Others: (Charm, Mind, Ki.......)
there is some overlapping but each kind of magic is way better in its area of specialization, other kind of effect are usable but only as higher level spells (such as +1 to +3 level for the same effect).
thus each spell-caster will need books to quickly change its readied spells and will have limits on the number of spells she can know, furthermore each kind of magic will give to the caster specific ability and organizations (to be further developed).
A game with a similar approach, no divine magic with spells, is the excellent Fantastic Heroes & Witchery.